1. Field of the Invention
The present invention relates to a nonaqueous electrolyte for improving overcharge safety and a lithium battery using the same.
2. Description of the Related Art
According to the development of advanced electronic devices, there is an increasing demand for small, lightweight portable electronic devices and equipment. Thus, there is a need for batteries having high energy density characteristics so as to supply a power to such devices and equipment, and research into such lithium batteries are being vigorously carried out.
A lithium battery comprises a cathode, an anode, an electrolyte for providing for a movement path of lithium ions and a separator disposed between the cathode and the anode. Lithium batteries produce an electrical energy by intercalation/deintercalation of lithium ions during oxidation and reduction occurring at the cathode and the anode. However, in the case where a battery is overcharged due to malfunction of a charger, for example, causing a sharp rise in voltage, excess lithium is precipitated at the cathode and excess lithium is intercalated into the anode. If both the cathode and the anode are thermally unstable, an organic solvent of the electrolyte is decomposed to cause rapid heat generation, such as thermal runaway, adversely affecting safety of the battery.
To overcome the foregoing disadvantage, various attempts to suppress overcharge of a battery by changing the composition of an electrolyte or adding additives to the electrolyte have been proposed in the field of lithium ion batteries. For example, U.S. Pat. No. 5,580,684 discloses a method of improving safety of a battery by increasing a self-extinguishing property of an electrolyte by adding phosphoric acid esters, such as trimethyl phosphate, tri(trifluoroethyl)phosphate or tri(2-chloroethyl)phosphate, to the electrolyte.
U.S. Pat. No. 5,776,627 discloses a method of enhancing battery safety by preventing migration of lithium by adding some additives such as thiophene, biphenyl or furan to be polymerized in the event of failure of a battery, and by allowing a safety vent of the battery to be easily opened by the gas produced by the additives.
Similarly, methods for enhancing battery safety are disclosed in U.S. Pat. Nos. 5,763,119, 5,709,968 and 5,858,573 in which an electrolyte includes additive such as 1,2-dimethoxy-4-bromo-benzene, 2-chloro-p-xyline and 4-chloro-anisol, and 2,7-diacetyl thianthrene, respectively.
Also, Japanese Patent Laid-Open Publication No. Hei 7-302614 discloses a battery protecting method in which overcharge current is consumed by forming a polymer using a benzene compound.
However, such conventional additives may be polymerized even under normal operating conditions or produce a large amount of gas due to oxidative decomposition, resulting in swelling of a battery. Also, use of such conventional additives may deteriorate various battery performance such as formation, standard capacity or cycle life characteristics.